Filtration is typically performed to separate, clarify, modify and/or concentrate a fluid solution, mixture or suspension. In the biotechnology and pharmaceutical industries, filtration is vital for the successful production, processing, and testing of new drugs, diagnostics and other biological products. For example, in the process of manufacturing biologicals, using animal cell culture, filtration is done for clarification, selective removal and concentration of certain constituents from the culture media or to modify the media prior to further processing. Filtration may also be used to enhance productivity by maintaining a culture in perfusion at high cell concentration. The invention provides an improved means for fractionating a mixture or suspension of molecules or particulates based on physical and/or chemical properties.
Several specialized filters and filtration methods have been developed to separate materials according to their chemical and physical properties. Filters that have been developed in the art include flat surface filters, pleated filters, multi-unit cassettes, and tubular forms such as hollow fibers. However, many of these filters have short operating lives, and when used to filter cell culture suspension or other biological fluids, they tend to clog with dead cells, cell debris, aggregates or other constituents of the fluid.
Animal cells grow substantially slower than most microorganisms, and lacking protective cell wall, they are also more fragile. Some known methods for increasing the productivity of microbial culture production, including increasing agitation rates and vigorous delivery of gases into the culture, are not feasible with animal cells. Thus, production is limited to very gentle culture conditions and low cell concentrations. One way to increase the cell concentration while maintaining gentle culture conditions is through the perfusion method.
In the perfusion method for growing cells, culture medium, whose nutrients have been consumed and which contains increased levels of harmful waste products, is continuously removed from the culture and replaced with fresh medium. The constant addition of fresh medium while eliminating waste products provides the cells with the nutrients required to achieve high cell concentrations. Unlike the constant changing conditions during the batch culture method of production, the perfusion method offers the means to achieve and maintain a culture in steady state.
In normal batch culture production processes, cells are first inoculated into a fresh medium and the cells rapidly enter a log grow phase. As they consume the medium nutrients and waste products accumulate, the cells transition to a stationary phase followed by a decay phase. While several methods have been developed to optimize batch culture production, in each case, these processes undergo rapid growth and decay cycles. In perfusion, however, since waste products generated by the culture are continuously removed and the culture is continuously replenished with fresh medium, it is possible to achieve a state of equilibrium in which cell concentration and productivity are maintained. Typically, about one culture volume is exchanged per day and the cell concentration achieved in perfusion are typically two to more than ten times that achieved at the peak of batch culture.
Filtration systems for biological fluids were described previously in the art. One type of external filtration perfusion system is, for instance, described in U.S. Pat. No. 6,544,424, which is incorporated herein by reference. This fluid filtration system comprises a fluid storage vessel connected to a filter-containing compartment that is connected to a diaphragm pump. The diaphragm pump alternatively aspirates the fluid out of the vessel through the filter and expels the fluid through the retentate end of the filter, back into the vessel. By doing so, the system creates an alternating tangential flow of fluid through the filter element.
A major drawback of this system is that the diaphragm pump contains moving parts that are prone to wastage and can often break during the process. When the diaphragm breaks, the filtration system is no longer closed and becomes susceptible for contamination. The filtration process must, therefore, be aborted, leading to high costs in the case of e.g., processes for production of biopharmaceuticals. Indeed, since processes for the production of biopharmaceutical molecules take several days, sometimes up to several weeks, the breakage of a pump during a process run would lead to high costs and long down time in a production facility. It takes up to days for getting a new process running. These perfusion cultures are mainly performed at the last stage of the production process, which means that a lot of time and money is wasted due to this failure. It usually takes three to five weeks before the stage of perfusion is reached. The costs involved can easily increase to more than over $100,000.
A second drawback is that the diaphragm pump described in U.S. Pat. No. 6,544,424 consists of a stainless steel jacket that contains a diaphragm usually made of rubber or silicone. Before each run, the elements of the diaphragm pump must be cleaned, assembled and sterilized. In addition, when the system is used for production of pharmaceutical products, the cleaning and sterilization procedures must be validated. That validation requires lengthy procedures and test runs, which are very costly and time consuming.
A third drawback of the systems currently in use is that they use invasive sensor technology where sensors are in contact with the product and those sensors cannot be replaced during the process.
The disclosure aims at providing improved fluid filtration systems that have less of, or even eliminate, these drawbacks.